1. Field of the Invention
The present invention relates to an image pickup apparatus which is suitable to obtain exposure information and focus adjustment information.
2. Description of the Prior Art
Conventionally, in this kind of apparatus, image pickup means such as an image pickup tube, semiconductor image pickup device and the like is scanned and driven synchronously with the standard television system and by integrating an output of at least a part of such image pickup means, photometric information is obtained or by detecting the state (for example, contrast state) of a part of the above-mentioned periodic scanning output, the information with respect to the focus adjustment state is obtained.
Such an apparatus has a drawback such that the above-mentioned photometric or distance metering information cannot be derived unless the field interval has one television field interval at shortest.
Such a drawback is not a large problem in ordinary video cameras and the like.
Namely, for example, when considering an iris as a system which is controlled, an auto iris for servo-controlling the iris in response to an output of the image pickup means is known in the conventional video camera. However, in the case where this auto iris is always closed-loop controlled, a servo error signal is small excluding an object which has a particularly remarkable change in brightness, so that it takes only a short time to effect control. Therefore, the drawback mentioned above will not become a large problem.
However, in the case where only one picture image is picked up by such an image pickup apparatus, this drawback becomes a large problem. Namely, to pick up only one picture image, the apparatus has to be constituted so as to turn the power supply immediately before use of the apparatus for the purpose of saving the power source.
In this constitution, however, it takes the time period corresponding to a few fields until the servo becomes stable at an object value since the above-mentioned servo loop has not been functioning prior thereto.
On the other hand, the amount of feedback cannot be set to a small value in consideration of the response speed of the system which is controlled, such as an auto iris and the like, and of the stability of the servo.
Also, when considering the point of accuracy of, e.g., AE (Auto Exposure), the dynamic rage of the image pickup apparatus, particularly, of the semiconductor image pickup device is much more narrow than that of the conventional silver halide camera, so that a higher degree of exposure accuracy is required.
Further, in case of silver halide films, it is possible to fairly correct a slight exposure error at the processing laboratory if there is such an error. However, in the case of the image pickup apparatus, it has to be avoided and the receiver side is adjusted whenever an exposure error is caused.
Thus, the failure of AE is not contrarily permitted.
In this way, in the image pickup apparatus to pick up one picture image, the AE and AF (Auto Focus) controls with excellent leading characteristics and high accuracy are needed. On the contrary, for example, another method is also considered whereby an AE control signal is quickly obtained by increasing the driving frequency of the clock generator to drive the image pickup device by several times.
However, this method has the drawback that since the horizontal transferring frequency of the image pickup device is remarkably high, the horizontal transfer itself can be hardly performed and the electric power consumption also increases in proportion to an increase in frequency.